The present invention relates to a magnetic head read/write preamplifier within a magnetic storage system. In particular, the present invention relates to a magnetic head swing clamp and cross-talk eliminator configured for interconnection with the read/write preamplifier.
A magnetic storage system includes a magnetic head that reads and writes information on a recording surface of a magnetic media, such as a magnetic disk. The magnetic head includes an inductive coil which reads and writes the information by sensing or creating a changing magnetic field. A read/write preamplifier is connected to the magnetic head at first and second head contacts. The preamplifier includes read circuitry and write circuitry for controlling the read and write operations.
The read/write preamplifier may be operated in either a read mode or a write mode. During read mode operation, the read circuitry monitors differential voltages at the head contacts created by current induced in the inductive coil by the changing magnetic field on the recording surface of the disk. The read circuitry includes first and second read data output terminals that supply read data output signals which are representative of the information stored on the disk.
During write mode operation, the write circuitry forces a relatively large write current through the inductive coil to create a magnetic field that polarizes bit positions on the recording surface. Digital information is stored by reversing the polarization of selected bit positions by reversing the direction of the current flow in the inductive coil.
In one typical storage system, the read circuitry includes a read differential transistor pair formed by first and second NPN-type transistors. The base terminals of the read pair are connected to the first and second head contacts, respectively. The emitters of the read pair are connected together at a read current sink. The collectors of the read pair form the first and second read data output terminals. The read data output signals at the first and second read data output terminals are applied to a read amplifier.
The write circuitry includes an H-switch write control circuit connected across the inductive coil at the first and second head contacts. The H-switch includes an upper write transistor pair and a lower write transistor pair that have control terminals which are operated by write data signals. The H-switch directs the write current through the inductive coil in either a first direction or a second direction, opposite to the first direction, as a function of the write data signals.
The read/write preamplifier configuration described above is well known in the art and is commonly used within magnetic storage systems. However, several problems arise with this configuration. First, fairly large voltage swings are generated at the head contacts during write mode operation for switching the direction of current flow quickly in the inductive coil. Because current in an inductor cannot change instantaneously, the voltage swings have a tendency to momentarily rise above their forcing voltage causing a voltage spike.
Such a condition can produce voltages across the connected read differential transistor pair which exceed the breakdown voltages of the transistors and degrade or otherwise damage the transistors. For example, if the first head contact rises greatly positive such that the voltage across the read pair is greater than the base-emitter voltage drop of the first transistor in the pair plus the emitter-base breakdown voltage of the second transistor in the pair, the second transistor in the pair may break down and degrade.
One common solution to the breakdown problem involves slowing down the H-switch such that voltage spikes do not occur. The base voltages of the connected read differential transistor pair are adequately controlled to prevent breakdown. This solution, however, limits the rate at which data may be written to the magnetic disk.
Another common solution to the breakdown problem includes the addition of diodes connected in series with the emitters of the read differential transistor pair. The diodes significantly increase the voltage at which the transistors will break down. This solution results in increased noise in the read data output signals.
The second problem that arises with the described read/write preamplifier configuration is cross-talk from one magnetic head to another. In larger storage systems, a plurality of read/write preamplifier circuits are connected together in parallel. The individual preamplifier circuits are commonly known as head cell circuits. There is one head cell circuit for each magnetic head in the storage system. Because only one head cell circuit and associated head are selected at a given time, cross-talk between a selected head and an unselected head is undesirable.
The read data output terminals of each head cell circuit are connected together at the read amplifier. Therefore, all of the read differential transistor pair collectors are connected together in parallel. Each transistor in the read pairs includes a parasitic capacitance across its collector-base junction. Large and rapid voltage transients at the head contacts in the selected head cell charge and discharge the read pair collector-base junction capacitance causing large currents at the read data output terminals. These currents can cause capacitive coupling between the collector-base junction at the selected head and the collector-base junctions at the unselected heads. As a result, the capacitive coupling may generate large currents through unselected magnetic heads, thereby writing through the read pairs of the unselected heads and corrupting data stored on the magnetic disk.
One common solution to the cross-talk problem includes connecting the collectors of the parallel read differential transistor pairs together through individual cascode transistors, thus reducing the capacitive coupling between the magnetic heads. This solution requires a large number of transistors which increases expense.
It is evident that there is a continuing need for improved solutions to both the read differential transistor pair breakdown problem and the cross-talk problem in magnetic storage systems.